Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Partners' Capital [Abstract]
Net income	$ 73,077	$ 66,704	$ 145,240	$ 127,892
Other comprehensive income (loss), before tax:
Change in net unrealized gain/loss on securities available for sale during the period	(113,268)	45,112	(78,741)	66,543
Change in net unrealized gain on securities transferred to held to maturity	(8,207)	(9,175)	(16,094)	(18,373)
Reclassification adjustment for net (gains) losses included in net income	0	(2)	(228)	(2)
Total securities available for sale and transferred securities, Before Tax Amount	(121,475)	35,935	(95,063)	48,168
Defined-benefit post-retirement benefit plans:
Change in the net actuarial gain/loss	1,749	671	3,498	1,343
Derivatives:
Reclassification adjustment for gains on interest rate swaps on variable-rate loans included in net income	0	(9,345)	0	(18,690)
Total other comprehensive income (loss), before tax	(119,726)	27,261	(91,565)	30,821
Deferred tax expense (benefit) related to other comprehensive income	(41,904)	9,541	(32,048)	10,787
Other comprehensive income (loss), net of tax	(77,822)	17,720	(59,517)	20,034
Comprehensive income (loss)	$ (4,745)	$ 84,424	$ 85,723	$ 147,926